The subject matter disclosed herein concerns improvements to gas turbine systems used in mechanical drive applications. In particular, but not exclusively the disclosure concerns gas turbine systems for driving compressors, e.g. compressors for refrigerant fluids in liquefied natural gas facilities.
The disclosure further concerns improvements in methods for operating a system comprising a gas turbine and a load, e.g. a compressor for LNG, or for oil and gas applications, a pump or other rotary equipment.
Liquefied Natural Gas (LNG) results from a liquefaction process, in which the natural gas is cooled using one or more refrigeration cycles in a cascade arrangement, until it becomes liquid. Natural gas is often liquefied for storage or transportation purposes, e.g. if pipeline transportation is not possible or economically unfeasible.
Cooling of the natural gas is performed using closed or opened refrigeration cycles. A refrigerant is processed in a compressor or compressors, condensed and expanded. The expanded, chilled refrigerant is used to remove heat from the natural gas flowing in a heat exchanger.
Refrigerant compressors in LNG, compressors for pipeline applications or other rotary equipment for applications in the oil and gas industry, are often driven by gas turbines. The gas turbine power availability (i.e. the power available on the turbine power shaft) is dependent upon ambient conditions, e.g. air temperature, and other factors, such as ageing. The turbine power availability increases with decreasing temperatures and, conversely, decreases with increasing temperatures. This causes power availability fluctuations both in the 24 hours as well as during the year, due to daily and seasonal temperature fluctuations.
It has been suggested to provide an electric motor in combination with a gas turbine to drive a load, comprised of e.g. one or more compressors. The electric motor is operated to supplement mechanical power to the load, to maintain the overall mechanical power on the load shaft constant, when power availability of the turbine decreases and/or to increase the total mechanical power used to drive the load. This function of the electric motor is referred to as helper duty. The same electric motor is usually used also as a starter motor, to accelerate the string formed by the gas turbine and the load from zero to the rated speed.
When an excess mechanical power is generated by the turbine, e.g. if the ambient temperature drops below the design temperature and consequent increase in power availability of the turbine, the excessive mechanical power generated by the gas turbine is converted into electric power, using the electric helper motor as a generator.
FIG. 1 schematically illustrates a system comprising a gas turbine arranged for mechanical drive applications, i.e. for driving a load different from an electric generator, in particular for driving a compressor or compressor train. The system 101 comprises a gas turbine 103. The gas turbine is in turn comprised of a gas generator 105 and a power turbine 107. The gas generator 105 is comprised of a compressor 109 and a high-pressure turbine 111. The gas generator 105 comprises a gas generator rotor including the rotor 109R of the compressor 109 and the rotor 111R of the high-pressure turbine 111. The rotor 109R of the compressor 109 and the rotor 111R of the high-pressure turbine 111 are mounted on a common shaft and together form a gas generator rotor.
The compressor 109 compresses ambient air, which is delivered to a combustion chamber or combustor 113, where the compressed air is mixed with a liquid or gaseous fuel and the fuel/air mixture is ignited to generate combustion gas. The high-temperature and high-pressure combustion gas is partly expanded in the high-pressure turbine 111. Mechanical power generated by the gas expansion in the high-pressure turbine 111 is used to drive the compressor 109.
Hot and partly expanded gas exiting the high-pressure turbine 111 flows through the power turbine or low-pressure turbine 107. The combustion gas expands in the power turbine 107 to generate mechanical power made available on a load coupling shaft 115. The power available on the load coupling shaft 115 is used to drive into rotation a load globally labeled 117. The load 117 can comprise a compressor or a compressor train, for example. In the embodiment of FIG. 1 the load 117 comprises a double compressor 117A, 117B.
The rotor of the power turbine 107 is mechanically separated from, i.e. not torsionally coupled to, the gas generator rotor formed by the compressor rotor 109R and the high-pressure turbine rotor 111R.
The gas generator rotor is connected through a shaft 119 to an auxiliary reduction gear 121. The auxiliary reduction gear 121 has an input shaft 123 which is mechanically connected to an electric motor 125 operating as a starter. A torque converter 127 and optionally a clutch 129 are arranged between the starter 125 and the input shaft 123 of the auxiliary reduction gear 121.
The starter 125 is connected to an electric power distribution grid schematically shown at G.
The electric motor or starter 125 is used to start the gas turbine 103. Starting is performed by energizing the electric motor 125 and by driving into rotation at gradually increasing rotary speed the gas generator rotor through the torque converter 127. Once sufficient air flows through the compressor 109, the gas generator can be ignited by delivering fuel to the combustor 113. The combustion gases are conveyed through the power turbine 107 and the gas turbine 103 starts rotating the load 117. The torque converter 127 allows gradual acceleration of the gas turbine 103 while the electric motor 125 rotates at constant speed according to the grid frequency.
Reference number 131 indicates an electric motor, operating as a helper and arranged at the end of the string comprising the gas turbine 103 and the load 117, opposite the electric motor 125. The helper 131 converts electric power into mechanical power to drive the load 117 in combination with the gas turbine 103, for example when the power available from the gas turbine 103 drops, for instance due to increasing environment temperature.
The system 101 is complex and has a large footprint.